well 6870 is great already but some games you may not be able to max out, with a 6950 most of games will be.
Good thing i always think in the future if you maybe buy a 1920x1080 screen in a near future good that you have a good graphics card already, i would recommend going for a 6950 will be great for your system and you will be able to play games on very high at 1920x1080.
Quick note before this gets closed - if you don't plan on upgrading to 1080p or higher resolution, just get the 1.5GB vram version of whichever card you pick up. I think they're supposed to come in both 1.5GB and 3GB versions and if you can find a 1.5 GB version for cheaper then go ahead and pick that up - 1600x900 will not need 3 GB of vram.
there is no need to go over 1gb vram for playing at 1080p, there will be no performance changes, more than 1gb is recommended for higher resolutions screens and or multi monitor setups.